We recently observed that in magnesium (Mg) loaded rats, the proportion of filtered Mg recovered in the papilla was greater than unity, indicating an entry of this cathion prior to the bend of the loop. The observation raised several questions concerning the site of Mg input, the relative participation of the ascending limb of the loop and the collecting duct as sources of this cathion, finally the integration of the Mg into the general mechanisms of the countercurrent system. We intend to study these various aspects by: 1. comparing the relative concentration of Mg in the loop of Henle and in the adjacent vasa recta. 2. Measuring the proportion of filtered Mg recovered at two different sites of the same collecting duct, in order to detect any movement of Mg in or out of the tubular lumen. Both procedures will be performed in normal and Mg loaded young rats in which the papilla are still prominent in the pyelic cavity. The first step already done consisted of setting up the microtechniques for measuring Mg and inulin concentration of the collecting duct fluid which resembles urine and the vasa recta plasma which resembles peripheral blood plasma enriched with proteins. These techniques are reportd on the section "Annex I". The general preparation of these animals has already been described in previous publications. Clearances of inulin and Mg will performed hourly. In the first series of experiments, Mg and inulin will be measured in the loop of Henle and in two adjacent capillaries. The finding of an equal or higher Mg concentration in the vasa recta compared to the tubula fluid, will be compatible with the hypothesis of a diffusion of Mg into the descending limb through the interstitial tissue and vasa recta. An opposite finding will suggest that Mg enters the tubular fluid at a more proximal part of the descending limb or in the pars recta. In the second series of experiments, the measurement of inulin and Mg will enable us to calculate the portion of filtered Mg present at two levels of the collecting duct, and therefore to detect any reabsorption, secretion or the absence of any transport of Mg. We think that this global approach of Mg physiology in the papilla may help for a better understanding of the countercurrent mechanism.